Added ipaaca.exit() as (currently) a circumvention for normal exit.

It will call the teardown stuff then os._exit(...). Later, when the
backends are fixed, we could simply alias it to sys.exit again.

Usage:
 - use ipaaca.exit() at the end of every python script
 - do not forget to catch errors as well as KeyboardInterrupt and
   SystemExit (or their default handlers will hang at exit time, too)
 - you can call ipaaca.exit with ant int argument (like sys.exit etc.)